Planar light source

ABSTRACT

A planar light source has a light emitting source, and a light guide plate for receiving light beams emitted from the light emitting source at a front side thereof and changing direction of the light beams, discharging the light beams from an upper surface thereof. A first anisotropic diffusion surface is formed on a lower surface of the light guide plate, and a reflection member is provided on the first anisotropic diffusion surface. The first anisotropic diffusion surface is a hologram diffusion surface or a hairline diffusion surface.

BACKGROUND OF THE INVENTION

The present invention relates to a planar light source for a backlight which is, for example, provided for illuminating a liquid crystal display (LCD) panel from behind, and more particularly to a light source provided with a light guide plate, a light emitting source such as a light emitting diode (LED) disposed at a side of the light guide plate, and preferably, a light direction limiting means such as a diffusion panel disposed above the light guide plate. In such a light source, the direction of light emitted from the light emitting source is changed so that a luminous flux exits the light guide plate from the upper surface thereof, and preferably, the direction is further adjusted by the light direction limiting means.

There is known a backlight unit comprising a planar light source for illuminating such a display as an LCD provided in a mobile terminal device and laptop computer. Japanese Patent Application No. 2002-146589 (Japanese Patent Application Laid-Open No. 2003-337333) discloses such a backlight unit in FIG. 17, which is shown in FIGS. 12 a and 12 b. FIG. 12 a is a perspective view showing the backlight unit and FIG. 12 b is a sectional view of the backlight unit.

A backlight unit 120 comprises a pair of LEDs (light emitting diodes) 102, a light guide plate 101, a diffusion panel 103, a Py prism sheet 104, a Px prism sheet 105, a reflection plate 106, and a transmissive or semi-transmissive LCD 107. The plates 101, 103, sheets 104, 105 are stacked.

The light guide plate 101 is made of a transparent plastic and has an upper surface 101 a, lower surface 101 b and front side 101 c. The LEDs 102 are mounted on an LED substrate 102 b and disposed opposite the front side 101 c. The diffusion panel 103 is disposed above the upper surface 101 a of the light guide plate 101 and the reflection plate 106 is disposed below the lower surface 101 b.

The light emitted from the LEDs 102 enters the light guide plate 101 from the front side 101 c. The entered light is repeatedly reflected by the upper and lower surfaces 101 a and 101 b and advances through the light guide plate 101. The lower surface 101 b has a fine prism surface so that, in accordance with the principles of the reflection and refraction of light, a part of the light is reflected toward the upper surface 101 a while the rest is refracted and enters the reflection plate 106. On the surface of the reflection plate 106, the light is reflected so as to return to the light guide plate 101. The reflection plate 106 thus serves to increase the light utility efficiency.

The direction of the discharged light is arranged in a small range by the diffusion panel 103, and further arranged by the prism sheets 104 and 105 in the Y and X-directions, and finally arranged in the Z-direction. The light arranged in the Z-direction illuminates the LCD 107. Hence the light is transmitted through the liquid crystal under an optimum condition, thereby enabling a clear display of high S/N ratio.

However, there is a following problem in the conventional planar light source. Namely, since the light is reflected in various directions at the lower surface 101 b, not a small number of light beams enter the upper surface 101 a at an angle approximate to the critical angle as shown in FIG. 12 c. Such a light beam refracts at an angle approximate to 90 degrees to the normal, that is, almost horizontally. In such a case, the light beam does not enter the diffusion panel 103, or even if it does, the incident angle is so large that it is difficult to efficiently change the direction of the light beam and render it to exit from the diffusing plate 103 and enter into the Py prism sheet 104. Hence it is difficult to efficiently change the light entering the light guide plate 101 from the LED 102 into a sufficiently bright illuminating light.

In order to resolve the problem, there is proposed a light source shown in FIGS. 13 a to 13 c where the light guide plate 101 has an anisotropic diffusing surface 101 h having a plurality of longitudinal hairlines or holograms provided to cause anisotropic diffusion. In the illustrated example, a plurality of hairline prisms are formed on the lower surface 101 b. Other constructions of the light source are the same as that of the backlight unit 120 of FIG. 12.

The operation of the device is based on the well-known principle described on page 5 of U.S. Pat. No. 6,347,873, for example. More particularly, since the anisotropic diffusion surface 101 h is formed on the upper surface 101 a of the light guide plate 101 a, the incident angle of the light beam applied thereto becomes a desired angle larger than 90 degrees, far from the horizontal direction. Hence, a tapered diffracted light beam ψ01 is generated even in the case where the incident angle of the incident light beam is approximate to the critical angle. Thus the incident angle of the light beams entering the diffusion panel 103 is increased so that the light utility efficiency is improved and the brightness of the illuminating light is increased.

Meanwhile, the diffused light beam ψ01 is an anisotropically diffused light beam having a longer axis in the longitudinal direction of the anisotropic diffusion surface 101 h than in the lateral direction as shown by the section thereof in FIG. 13 c. The diffusion surface 101 h is thus adapted to have a large diffusion characteristic in the longitudinal direction thereof. The reason the anisotropic diffusion surface is so adapted is that although the diffusion caused by each of the numerous grooves overlaps each other in the lateral direction of the surface 101 h so that the light is sufficiently laterally diffused, in the longitudinal direction, it is necessary to render the diffusion at each groove wider in the longitudinal direction than in the lateral direction.

However, there is another problem even in the thus improved planar light source when examined more closely. Referring to FIGS. 13 b and 13 d which are enlarged diagrams showing the part adjacent the front side 101 c of the light guide plate 101, a light beam s01 reflected at the lower surface 101 b and a light s02 reflected at the upper surface 101 a are examined. The light beam s01 is refracted at the prism of the lower surface 101 b and further reflected by the reflection plate 106 so as to again reach the lower surface 101 b. The beam is refracted and enters the diffusion surface 101 h of the upper surface 101 a at an incident angle approximate to the critical angle but smaller. Accordingly, the beam exits the diffusion surface 101 h as a diffused light beam ψ01 at an angle within a predetermined range without reflecting. Since the upper surface 101 a is not smooth, the beam does not exit horizontally.

On the other hand, supposing the initial incident angle of the light beam s02 is larger than the critical angle, the beam enters the diffusion surface 101 h. Contrary to the case where the upper surface 101 a is smooth so that the incident light is 100 percent totally reflected, due to the diffusion, a quite a large part of the light exits as a secondary diffused light beam ψs as shown by the dotted lines in FIG. 13 b. The rest of the light beam is reflected and permeated through the lower surface 101 b, reflected at the reflection plate 106, refracted at the lower surface 101 b and reaches the upper diffusion surface 101 h. Due to the operation of the prism at the lower surface 101 b, the incident angle at the diffusion surface 101 h is gradually reduced each time the beam is reflected. In the example shown in the figure, in the light beam s02, after going through the reflection twice, the incident angle at the diffusion surface 101 h becomes smaller than the critical angle so that only a diffused light beam ψ02 is emitted. Since the intensity of the diffused light beam ψ02 decreases by that of the secondary light beam ψs every time the beam is reflected, the intensity of the diffused light beam ψ02 becomes smaller than that of the diffused light beam ψ01.

In addition, as shown in FIG. 13 d, the diffused light beams ψ01 and ψ02 are able to enter the diffusion panel 103 in ranges having widths W01 and W02, respectively. Since the distance between the light guide plate 101 and the diffusion panel 103 cannot be increased, it is impossible to enlarge the widths W01 and W02.

More particularly, as explained with regard to the light beams ψ01 and ψ02, regarding a single internal light beam, although diffused at the diffusing surface 101 h, the widths W01 or W02 of the range through which the beam reaches the diffusion panel 103 is small. In order to increase the range, it is necessary to let the light beams enter the diffusion surface 101 h at various incident angles so that the position of exit of the beams ψ01 and ψ02 vary. As the position is moved toward the right in the drawing, the number of reflections at the diffusion surface 101 h increases, which results in decrease in intensity of the light beam. Hence the luminance of the output light is decreased as the distance from the front surface 101 c increases toward the right side of the light guide plate in FIG. 13 b. Consequently, the intensity of incident light entering the diffusion panel 103 also decreases at the right side so that the distribution of luminance becomes uneven. The light transmitted through the diffusion panel 103 is adjusted in X-, Y-, and Z-directions by the Py prism sheet 104 and Px prism sheet 105 and becomes the illuminating light. However, the illuminating light also becomes uneven in luminance.

Namely, in the planar light source provided with the anisotropic diffusion surface on the upper surface of the light guide plate, the intensity of light which exits the diffusion surface becomes lower toward the right, that is, as the distance from the LED increases. As a result, the luminance of the light exiting the light guide plate and entering an optical path adjusting means such as the diffusion panel becomes uneven depending on the position. Such a tendency is retained even in light entering the LCD 107 after passing through the optical path adjusting means so that the quality of illumination is decreased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a planar light source wherein the luminance of the illuminating light is large and even.

According to the present invention, there is provided a planar light source comprising a light emitting source, a light guide plate for receiving light beams emitted from the light emitting source at a front side thereof and changing direction of the light beams, discharging the light beams from an upper surface thereof, a first anisotropic diffusion surface formed on a lower surface of the light guide plate, and a reflection member provided to face the lower surface of the light guide plate on which the first anisotropic diffusion surface is formed.

The reflection member is provided for reflecting the light beams emitted from the first anisotropic diffusion surface to return the light beams to the first anisotropic diffusion surface to form anisotropic diffused light beams which are increased in sectional area by two diffusion.

The first anisotropic diffusion surface is a hologram diffusion surface or a hairline diffusion surface.

A plurality of prisms are formed on the upper surface of the light guide plate.

The angle between the upper surface and the front side of the light guide plate is 90 degrees, and the thickness of the light guide plate is gradually reduced toward a rear side, and the first anisotropic diffusion surface is formed on the lower surface.

In another aspect of the present invention, a second anisotropic diffusion surface is formed on a part of the upper surface of the light guide plate and the first anisotropic diffusion surface comprises two kinds of anisotropic diffusion surfaces which are different from each other in characteristic of diffusion, and further comprises a diffusion plate and light direction correcting means provided on the light guide plate.

The light direction correcting means comprises a Px prism sheet having X-direction prisms and a Py prism sheet having Y-direction prisms.

These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a is a perspective view showing a planar light source according to a first embodiment of the present invention;

FIG. 1 b is an illustration showing a light guide plate provided in the light source of FIG. 1 a;

FIGS. 1 c and 1 d show sections of diffused light beams taken along lines I-I and II-II of FIG. 1 b, respectively;

FIG. 2 is a perspective view of a light guide plate provided in the planar light source of FIG. 1 a;

FIG. 3 is a sectional view of the planar light source explaining the operation thereof;

FIG. 4 is an illustration showing a part of a light source according to a second embodiment of the present invention;

FIG. 5 is an illustration showing a part of a light source according to a third embodiment of the present invention;

FIG. 6 is an illustration showing a part of a light source according to a fourth embodiment of the present invention;

FIG. 7 a is a diagram schematically explaining an anisotropic diffusion in the present invention;

FIG. 7 b shows a section of a diffused light beam shown in FIG. 7 a;

FIG. 7 c shows orthogonal coordinates of X-, Y- and Z-directions;

FIG. 8 a to 8 e are diagrams explaining the principle of the anisotropic diffusion caused by a hologram;

FIGS. 9 a to 9 d are diagrams explaining the operation of the anisotropic diffusion caused by a hologram provided on a surface of a light guide plate of the present invention;

FIGS. 10 a to 10 d are diagrams explaining the operation of the anisotropic diffusion by minuscule prisms provided on a surface of a light guide plate of the present invention;

FIGS. 11 a to 11 d are diagrams explaining the operation of the anisotropic diffusion by small prisms provided on a surface of a light guide plate of the present invention;

FIG. 12 a is a perspective view showing a conventional light source;

FIGS. 12 b and 12 c are illustrations explaining the operation of the light source of FIG. 12 a;

FIG. 13 a is a perspective view of a light guide plate provided in another conventional light source;

FIG. 13 b is an illustration explaining the operation of the light guide plate of FIG. 13 a;

FIG. 13 c shows a section of a diffused light beam taken along a line XIII-XIII of FIG. 13 b; and

FIG. 13 d is an illustration explaining the operation of the light guide plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A principle of the anisotropic diffusion by holograms and hairlines will be described hereinafter.

FIGS. 7 a to 7 c are diagrams conceptually explaining the anisotropic diffusion. Referring to FIG. 7 b, when a coherent light beam s from a laser light emitting source L enters an anisotropic diffusion surface H in a perpendicular direction with respect to the X-Y plane illustrated in FIG. 7 c, a diffused light beam ψ is generated. As shown in FIG. 7 a which shows a section of the beam taken along a line VII-VII of FIG. 7 b, the beam is anisotropic in shape in that the beam has a long axis in the X-direction and a short axis in the Y-direction. Namely, the diffused light ψ is an anisotropic diffused light diffused more in the X-direction than in Y-direction.

An example of a hologram for producing such an anisotropically diffused light beam is described. A hologram records phase information of standing waves which are generated when an object beam and a reference beam interfere with each other. Accordingly, upon entrance of a reference beam, the hologram reproduces a diffracted light beam as an object beam.

There is a type of a hologram called “surface relief hologram” where the phase information of the coherence light is recorded not in tones, but by bumps or pits formed by using photoresist on a surface of the hologram. FIG. 8 a shows a portion of the surface relief hologram for generating an anisotropically diffused light beam as an object beam.

Referring to FIG. 8 a, a hologram H has a plurality of pits Hd formed on a surface thereof thereby forming substantially flat portion Hk between the pits. Each of the pits Hd is elongated in the Y-direction and aligned in parallel in the X-direction. In the drawing, b1 is a distance between the adjacent pits Hd in the Y-direction and d1 is a distance between the centerlines of the adjacent pits Hd in X-direction.

As shown in FIG. 8 b, in the X-Z plane, when coherent incident light beams s perpendicularly enter the surface of the hologram H which is in the X-Y plane, namely, in the direction Z at an incident angle of zero degrees, some of the beams reach the pits Hd while the other reach the flat portion Hk. The light reaching the pits Hd are diffusely reflected so that hardly any of the light is transmitted through the hologram H. On the other hand, each of the light beams reaching the flat portion Hk becomes a point source so as to generate an elementary wave. Accordingly, a diffracted light beam is generated in a direction strengthening the elementary waves. A zeroth order diffracted light beam sd0 of the diffracted light beam is transmitted in the same direction as the incident light s so that the output angle of the incident light s is zero degrees. On the other hand, as shown in FIG. 8 b, there is a relationship between the output angle θ1 of a first order diffracted light beam sd1 and the distance d as described below. sin θ1=±λ/d 1  (1) wherein λ is a wavelength of the incident light beam s. It can be seen from the equation (1) that the output angle θ1 can be set between zero degrees and ±90 degrees by determining the distance d1 between the pits. As shown in FIG. 8 c where the diffusion shown in FIG. 8 b is enlarged, a single incident light beam s is diffused in the X-Z plane by the hologram H as a diffusing surface so as to become a sector shaped diffused light beam ψx having an angular width of 2θ1. In the case where the angular width becomes smaller than 2θ1, the light can be sufficiently diffused by varying the distance d1 between the pits as appropriate.

Referring to FIG. 8 d, in the Y-Z plane, when the incident light beams s perpendicularly enter the surface of the hologram H, the beams passing though the flat portion Hk at the center with regard to the direction Y between adjacent pits Hd advance straight whereas the beams at the end portions are diffracted in Y-direction only at a diffraction angle θy. The value of the diffraction angle θy depends on the wavelength λ and the distance b1 as follows. sin θy=±λ/b 1  (2) Comparing the equations (1) with (2), it can be understood that if the distance b1 between the pits is sufficiently larger than the distance d1, the diffraction angle θy can be sufficiently smaller than the output angle θ1. As shown in FIG. 8 e where the diffusion shown in FIG. 8 d is enlarged, a single incident light beam s is diffused by the hologram H so that a sector shaped diffused light beam ψy having an angular width of 2θy is generated in the X-Z plane.

The angular width 2θ1 of the diffused light in the X-Z plane shown in FIG. 8 c is much larger than an angular width 2ψy in the Y-Z plane in FIG. 8 e. Accordingly, the hologram H provides diffused light beams the angular widths of which differ depending on the direction of the sectional plane. Hence the hologram H can be said as having an anisotropic diffusion surface. The hologram H is hereafter referred to as an anisotropic diffusion surface H.

An example of the anisotropic diffusion where the anisotropic diffusion surface H is formed on a light guide plate in actual practice is described hereinafter with reference to FIGS. 9 a to 9 d.

Referring to FIG. 9 a, the anisotropic diffusion surface H having a phase hologram of a surface relief type shown in FIG. 8 a is formed on a light guide plate D. The pits and flat portions on the anisotropic diffusion surface H are formed by photoresist to represent tone of coherent light which corresponds to phase information. In order to form the pits and flat portions, a metal mold on which the shape of the diffusion surface is transferred is produced by plating and the metal mold is incorporated in a mold for manufacturing the light guide plate by injection molding.

Referring to FIG. 9 b, in the plane X-Z, when the incident light beams s enter the anisotropic diffusion surface H at an incident angle θ, the beam at each flat portion Hk becomes a point source so as to generate an elementary wave, thereby forming diffracted light in a direction strengthening the elementary waves. With regard to a zeroth order diffracted light beam sd0, sin θ0=nsin θ  (3) wherein θ0 is an output angle of the zeroth order diffracted beam light sd0 and n is the refractive index of the light guide plate D. With regard to the first order diffracted light beam sd1, sin θ1=nsin θ±(λ/d 1)=sin θ0±(λ/d 1)  (4) As shown by the equation (3), Snell's law is applied in the case of zeroth order diffracted light beam. As shown by the equation (4), the incident angle θ1 of the first order diffracted light beam varies within a range about the incident angle θ0 of the zeroth order diffracted light beam so that, as shown in FIG. 9 b, diffracted light beams sd1 ⁻ and sd1 ⁺ are generated. In the case where distance d1 is so small that λ/d1>2, |sin θ0±(λ/d 1)|>1 Hence the equation (4) cannot be satisfied. Accordingly, the zeroth order diffracted light beam sd0 exists but not the first order diffracted light beam sd1. Namely, if the distance d1 between the point sources is small, the light is diffracted only in accordance with the Snell's law as in the case of smooth surface and diffusion does not occur. Hence a distance of several times as that of the wavelength λ is generally selected as the distance d1 between the pits Hd.

As shown in FIG. 9 c where the diffusion shown in FIG. 9 b is enlarged, a single incident light beam s is diffused in the X-Z plane by the anisotropic diffusion surface H so as to become a sector shaped diffused light beam ψx which spread out between the first order diffracted light beams sd1 ⁺ and sd1 ⁻ with the zeroth order diffracted light beams sd0 at the center. The diffraction in the Z-Y plane is the same as that in FIG. 8 e. Thus in the present example, an anisotropic diffusion can be realized by forming the anisotropic diffusion surface H with a hologram on the surface by flat portions and pits which corresponds to the phase information of the coherent light.

When the incident angle θ of the incident light beam entering the anisotropic diffusion surface H exceeds the critical angle as shown in FIG. 9 d, the zeroth order diffracted light beam sd0 is reflected at an output angle which is equal to the reflection angle θ0 at total reflection. There is also generated a first order diffracted light as beams sd1 ⁺ and Sd1 ⁻ in accordance with the Snell's law and a second order diffracted light beam s2 ⁻ although equation thereof is not shown here. The light beams sd1 ⁻ and s2 ⁻ are transmitted through the anisotropic diffusion surface H and exit out as the diffused light beam ψx. Thus incident light is not 100 percent totally reflected even when the incident angle θ exceeds the critical angle but a part of the incident light is reflected while the remaining part is diffused out of the light guide plate as the secondary diffused light beam ψs shown in FIG. 13 b.

FIGS. 10 a to 10 d show another example of the anisotropic diffusion where the light guide plate D has an anisotropic diffusion surface Hp on which a plurality of miniscule prisms ph arranged in hairlines in the X-direction are formed. A distance d2 between the adjacent prisms ph and a length b2 of each prism are about the same as the distance d1 and the distance b1 between the pits, respectively, on the anisotropic diffusion surface H in FIG. 9 a. When incident light beams s enter the hairline diffusion surface Hp at an incident angle θ, each beam reaching the inclined surface of the prism ph becomes a point source so as to produce an elementary wave, thereby forming diffracted light in a direction strengthening the elementary waves. With regard to the incident angle of the zeroth order diffracted light beam sd0, the aforementioned equation (3) is satisfied, and with regard to the incident angle of the first order diffracted light beam sd1, the equation (4) is satisfied. Namely, if the distance between the miniscule prisms are on the order of several times as that of the wavelength λ, the anisotropic diffusion will occur in the same manner as with the anisotropic diffusion surface H having a diffraction grating shown in FIG. 9 a regardless of the inclination of the prisms. Thus the diffused light beams ψx is generated as shown in FIG. 10 c.

Referring to FIG. 10 d, when the incident light beam s enters the anisotropic diffusion surface Hp formed on the light guide plate D from ambient atmosphere at an incident angle θ, the zeroth order diffracted light beam sd0 and the first order diffracted light beam sd1 are diffused within the light guide plate D. In accordance with the Snell's law, there is the following relationship between the incident angle θ and the output angle θ0. sin θ0=(sin θ)/n  (5) The relationship between the first order diffracted light beam sd1 and the output angle θ1 is, sin θ1=[sin θ±(λ/d 2)]/n=sin θ0±(λ/nd 2)  (6) Namely, as shown in FIG. 10 d, if the light beam s enters the opposite side of the light guide plate D from the ambient atmosphere, there is produced a sector shaped light beam ψx in the X-Z plane where the first order diffraction light spread out between beams sd1 ⁺ and sd1 ⁻. Thus the anisotropically diffused light is generated as in the case shown in FIG. 10 c.

FIGS. 11 a to 11 d show another example of the light guide plate D having the anisotropic diffusion surface Hp having hairline prisms ph. In the example, the distance d2 between the prisms ph is considerably larger than the wavelength λ of the light beams. In accordance with the equation (4), the output angle θ1 of the first order diffracted light beam caused by the interference of the light beams exiting the adjacent prisms ph is substantially equal to the output angle θ0 of the first order diffracted light beam so that the first order diffracted light beam sd1 is considered as not to exist. Regarding the output angle θ0 of the zeroth order diffracted light beam sd0, the Snell's law is established not with regard to the hairline anisotropic diffusion surface Hp but with regard to an inclined surface ps of the prism ph. The same can be said for ordinary large prisms. However, in the case of prisms of middle size between the large and miniscule prisms, there occurs diffusion at a diffraction angle θr at the end portions of the inclined surface ps as shown in FIGS. 11 a and 11 c. The diffraction angle θr has the same characteristics as the diffraction angle θy in the equation (2) and decreases as the length of the inclined surface ps is increased. As shown in FIG. 11 a, an inclination angle β of the inclined surface ps is equal to the incident angle θ and the incident light beam s enters the inclined surface ps perpendicularly. Thus the incident angle θ is equal to the output angle θ0 of the zeroth order diffracted light beam sd0. Accordingly, diffracted light beams sdr each diffracted at the angle θr are generated about the diffracted light beam sd0. The diffracted light beams sd0 and sdr form a diffused light beam ψx having a sector shape when enlarged as shown in FIG. 11 b is thus generated.

When the inclination angle β of the prism is increased as shown in FIG. 11 c, the direction of the zeroth order diffracted light beam sd0 is bent to the left by the refraction so as to become approximately perpendicular to the hairline anisotropic diffusion surface Hp and the diffracted light beams sdr which are diffracted at only diffraction angle θr are diffused about the zeroth order diffracted light beam sd0, thereby forming a sector shaped diffused light ψx when enlarged in FIG. 11 d. Thus, the diffusion range is enlarged to the left side diverged from the extension of the incident light beam s.

Although it has been explained in the above description with regard to FIGS. 11 a to 11 d that the extent of the diffusion depends on the diffraction angle θr, by setting the angle β of the inclined surface ps of the prism ph at various angles, the output angle θ0 of the zeroth order diffraction light sd0 can be varied. Accordingly, the angular width of the emitted light can be increased even though the diffraction angle θr may be small.

The anisotropic diffusion surface used in the present invention need not be limited to surfaces having holograms and hairlines but may include other surfaces capable of causing anisotropic diffusion such as those having shades or uneven surface in a shape of a plurality of ellipses, parabola, and curves such as hyperbola.

The first embodiment of a planar light source of the present invention is described hereinafter with reference to FIGS. 1 a to 1 d, FIG. 2, and FIG. 3. FIG. 1 a is a perspective view showing the entire light source, FIG. 1 b is an illustration showing the operation of a light guide plate provided in the light source, FIGS. 1 c and 1 d are diagrams describing the operation of the planar light source, FIG. 2 is a perspective view of a light guide plate provided in the light source, and FIG. 3 is a sectional view of the planar light source explaining the illuminating operation thereof.

Referring to FIG. 1 a, a planar light source 20 comprises a pair of LEDs 2, a light guide plate 1, a diffusion panel 3, a Py prism sheet 4, a Px prism sheet 5, and a reflection plate 6. The reflection plate 6, light guide plate 1, diffusion plate 3, prism sheets 4 and 5 are stacked.

The light guide plate 1 is made of a transparent plastic and has an upper surface 1 a, lower surface 1 b and front side 1 c. The LEDs 2 are mounted on an LED substrate 2 b and disposed opposite the front side 1 c. A plurality of prisms 1 p are formed on the upper surface 1 a. Each prism 1 p has a polished mirrored surface. As shown in FIG. 2, which shows the light guide plate as seen from the underside thereof, the light guide plate 1 has, on the underside thereof, a hologram diffusion surface or a plurality of fine hairline grooves such as those shown in FIG. 10 a so as to form an anisotropic diffusion surface 1 h on the lower surface 1 b.

The diffusion panel 3 is disposed above the upper surface 1 a of the light guide plate 1 and the reflection plate 6 is disposed to face the lower surface 1 b of the light guide plate on which the anisotropic diffusion surface 1 h is formed.

The light emitted from the LEDs 2 enters the light guide plate 1 from the front side 1 c. The entered light is repeatedly reflected by the prisms 1 p on the upper surface 1 a and the anisotropic diffusion surface 1 h on the lower surface 1 b, and as shown in FIG. 1 a, exits the light guide plate 1 through various paths.

Optical paths of some light beams selected from the various incident light beams emitted from the LED are described as a representative example with reference to FIG. 1 b which shows the planar light source 20 in detail. Of the incident light beams applied to the light guide plate 1, a light beam s1 which is applied to the anisotropic diffusion surface 1 h formed on the lower surface 1 b and a light beam s2 which is applied to the prism surface 1 p formed on upper surface 1 a are examined. The light beam s1 is diffused at a diffusion angle determined by the miniscule structure of the anisotropic diffusion surface 1 h so as to emit a diffused light beam ψ11. The diffused light beam ψ11 is reflected at the reflection plate 6 and again enters the anisotropic diffusion surface 1 h. The diffused light beam ψ11 is further diffused so that a diffused light beam ψ12 is emitted and enters into the light guide plate 1. FIG. 1 c shows a section of the diffused light beam ψ11 taken along a line I-I of FIG. 1 b and FIG. 1 d shows a section of the diffused light beam ψ12 taken along a line II-II of FIG. 1 b. It can be seen from FIGS. 1 c and 1 d that the anisotropic property is more enhanced in the re-diffused light beam ψ12 than in the first diffused light beam ψ11.

By repeating the diffusion twice, the anisotropic property, that is, the tendency to diffuse in the direction of the hairlines, is enhanced. When seen in the X-Z plane, the width of the finally diffused light beam ψ12 is increased in the course of the optical path length from the lower surface 1 b to the prism 1 p. The light beam ψ12 is refracted by and transmitted through the prism 1 p so as to enter the diffusion panel 3 at a width W12 as shown in FIG. 1 b. The incident width W12 is much larger than the width W01 of the diffused light beam ψ01 entering the diffusion panel of the conventional planar light device 101 shown in FIG. 13 c. In FIG. 1 b, a light beam ψ01 and the width W01 of the conventional device are shown only as a comparison and not as the operation of the present embodiment.

On the other hand, the light beam s2 enters the prism 1 p on the upper surface 1 a and is totally reflected. The reflected light enters the anisotropic diffusion surface 1 h at the lower surface 1 b without losing any light intensity and then becomes a diffused light beam ψ21. The diffused light beam ψ21 is reflected at the reflection plate 6 and re-enters the diffusion surface 1 h. The diffused light beam ψ21 is further diffused to become a diffused light beam ψ22 which is led inside the light guide plate 1. The diffused light beam ψ22 is refracted at the prism 1 p on the upper surface, exits the surface and enters the diffusion panel 3. The diffused light beam is increased in width during the course of the optical path length so that the beam ψ22 enters the diffusion panel in a range of a sufficiently large width W22. Unlike the light beam s02 entering the upper surface in the conventional device shown in FIG. 13 b, since the flat surface of the prism 1 p allows the total reflection, the intensity of the light beam s2 reflected at the upper surface 1 a is not reduced. Accordingly, the diffused light beam ψ22 having a sufficient luminance permeates the upper surface 1 a and enters the diffusion panel 3. Although not illustrated in the drawing, in the case of a beam which is reflected several times at the upper surface 1 a and reaches the upper surface 1 a approximate the right end of the light guide plate 1, the luminance is not weakened through the reflections so that a flux of light having sufficient luminance is refracted by the prism surface, transmitted through the surface and directed toward the diffusion panel 3.

Accordingly, in the present embodiment, the light can be emitted in a wide range at incident widths larger than in the conventional device without causing decrease in intensity. The light beams uniform in luminance enter into the diffusion panel 3 at a predetermined angular width, and are thereafter transmitted through the diffusion panel 3, Px prism sheet 4, Py prism sheet 5 in order, so that the beams efficiently changes the directions to become an illuminating light in Z or vertical direction without losing the intensities thereof. Accordingly, the light can illuminate the transmissive or semi-transmissive LCD 7 shown in FIG. 3 with high and even luminance.

The second embodiment of the planar light source of the present invention is described hereinafter with reference to FIG. 4 showing the light guide plate 1 in detail. On the lower surface 1 b of the light guide plate 1, in addition to the anisotropic diffusion surface 1 h, there is formed a second anisotropic diffusion surface 1 h 2 adjacent the front side 1 c. The characteristic of the anisotropic diffusion surface 1 h 2 differs from that of the anisotropic diffusion surface 1 h in that the directions toward which the diffused beams are emitted differ. Namely, the second anisotropic diffusion surface 1 h 2 has a miniscule structure shown in FIG. 11 c, for example. A light beam s3 entering the front side 1 c and exiting the light guide plate 1 from second anisotropic diffusion surface 1 h 2 is diffused in both sides of the normal of the anisotropic diffusion surface 1 h 2 and become a diffused light beam ψ31. The diffused light beam ψ31 is applied to the reflection plate 6, reflected, again permeates the second anisotropic diffusion surface 1 h 2 and reaches the light guide plate 1 as a diffused light beam ψ32. The light beam ψ32 is refracted at the upper surface 1 a of the light guide plate 1 and enters the diffusion panel 3. Since the diffused light beam ψ32 enters the light guide plate 1 at a position close to the front side 1 c at the left side of the light guide plate, the refracted light beam sufficiently covers the left edge area of the diffusion panel 3. The other constructions and operation are the same as those of the first embodiment shown in FIGS. 1 a through 1 d.

In accordance with the second embodiment, the light beams, the output angles of which are limited within a desired angle, enter the entire area of the diffusion panel 3 so that the luminance of the illuminating light emitted from the planar light source is improved while maintaining the level of brightness.

Referring to FIG. 5, showing the third embodiment of the present invention, a light guide plate 11 has a flat horizontal upper surface 11 a and a lower surface 11 b inclining upward as the distance from a front side 11 c increases. The lower surface 11 b has a first anisotropic diffusion surface 11 h and a second anisotropic diffusion surface 11 h 2 adjacent the front side 11 c having the same characteristics as the second anisotropic diffusion surface 1 h 2 of the second embodiment. A third anisotropic diffusion surface 11 h 3 is formed on the upper surface 11 a at an appropriate position adjacent the front side 1 c. The third anisotropic diffusion surface 11 h 3 has the same characteristics as the anisotropic diffusion surface 11 h.

In operation, of the light beams emitted from the LED 2, one of the light beams, a light beam s31, is directly applied to the third anisotropic diffusion surface 11 h 3 so that a diffused light beam ψ41 within a predetermined angle range exits the upper surface 11 a of the light guide plate 11. On the other hand, another beam s32 which strikes the upper surface 11 a at the right side portion of the third anisotropic diffusion surface 11 h 3 is totally reflected so as to advance downward and enters the first anisotropic diffusion surface 11 h. The beam is diffused and enters the reflection plate 6 , reflected and re-enters the anisotropic diffusion surface 11 h. As a result, a diffused light beam ψ42 is emitted and refracted by the upper surface 1 a and enters the diffusion panel 3 at a large width.

Another light beam s33 reaches the anisotropic diffusion surface 11 h on the lower surface 1 b and is diffused and reflected at the reflection plate 6 to produce a diffused light beam ψ43. The diffused light beam ψ43 is refracted at the upper surface 1 a, thereby entering the diffusion panel 3 at a wide width.

Although the light beam s32 is subjected to a total reflection at the upper surface 1 a before becoming the diffused light beam ψ42, since the intensity of the light is not reduced by the total reflection, the same intensity as that of the diffused light beam ψ43 which had not undergone the total reflection is maintained.

In the case of another light beam s34 which is directly applied to the second anisotropic diffusion surface 11 h 2 on the lower surface 11 b, a diffused light beam ψ44 is diffused at a portion adjacent a front surface 11 c in accordance with the principle described in the second embodiment with regard to the second anisotropic diffusion surface 1 h 2. The diffused light beam ψ44 is refracted at the upper surface 11 a so as to sufficiently cover the left side portion of the diffusion panel 3. Thus, the diffused light enters a wide area of the diffusion panel 3. Accordingly, the light source of the present embodiment also emits uniform and bright illuminating light.

The fourth embodiment of the present invention shown in FIG. 6 is similar to the third embodiment shown in FIG. 5 except that the third anisotropic diffusion surface 11 h 3 is omitted in the fourth embodiment. When the incident light beam s31 enters the upper surface 11 a at an approximate but smaller angle than the critical angle, the light beam s31 is refracted and emitted as an output light beam s311 at an output angle of about 90 degrees. Hence the light utility efficiency at the diffusion panel 3 is reduced. The third embodiment prevents such a problem. However, since there is only a few internal light beams under such a condition, a similar effect as that of the third embodiment is obtained even in the light source of FIG. 6 without the third anisotropic diffusion surface. Thus the fourth embodiment also provides a planar light source which emits a bright and even illuminating light.

In accordance with the present invention, the anisotropic diffusion surface such as those formed by hairlines and holograms is provided not on the upper surface but on the lower surface of the light guide plate, and the reflecting means such as the reflection plate is provided opposite the anisotropic diffusion surface. Accordingly, the following effects are obtained.

(1) The light beam which is emitted from the light emitting source such as LED and led into the light guide plate and to the lower surface thereof is diffused by the anisotropic diffusion surface at a predetermined angle, reflected by the reflecting means, and re-diffused at the anisotropic diffusion surface. Since the diffusion occurs twice interposing a reflection at the reflecting means, the anisotropic property of the beam is enhanced. Due to the two diffusions and increase in optical path length, the range of the beam reaching the upper surface of the light guide plate is increased.

(2) Most of the light beams which enter the upper surface of the light guide plate directly, or through reflection at the lower surface, are, due to the total reflection, applied to the anisotropic diffusion surface on the lower surface, and go through diffusion, reflection and re-diffusion as the light beams described above. These light beams reach another large area of the upper surface diverged from the above described area while maintaining sufficient intensity of light.

(3) Thus, the light beams enter substantially the entire area of the upper surface of the light guide plate at a desired incident angle by the use of anisotropic diffusion surface. Thereafter, the beams are refracted and emitted upward.

As a result, when the output light beams from the light guide plate are controlled to vertically advance using diffusion panel and other optical path adjusting means, the light utility efficiency is increased. Hence a light source enabling to emit illuminating light which is bright and even in luminance is provided.

While the invention has been described in conjunction with preferred specific embodiment thereof, it will be understood that this description is intended to illustrate and not limit the scope of the invention, which is defined by the following claims. 

1. A planar light source comprising: a light emitting source; a light guide plate for receiving light beams emitted from the light emitting source at a front side thereof and changing direction of the light beams, discharging the light beams from an upper surface thereof; a first anisotropic diffusion surface formed on a lower surface of the light guide plate; and a reflection member provided to face the lower surface of the light guide plate on which the first anisotropic diffusion surface is formed.
 2. The planar light source according to claim 1 wherein the reflection member is provided for reflecting the light beams emitted from the first anisotropic diffusion surface to return the light beams to the first anisotropic diffusion surface to form anisotropic diffused light beams which are increased in sectional area by two diffusion.
 3. The planar light source according to claim 1 wherein the first anisotropic diffusion surface is a hologram diffusion surface.
 4. The planar light source according to claim 1 wherein the first anisotropic diffusion surface is a hairline diffusion surface.
 5. The planar light source according to claim 1 wherein a plurality of prisms are formed on the upper surface of the light guide plate.
 6. The planar light source according to claim 1 wherein the angle between the upper surface and the front side of the light guide plate is 90 degrees, and the thickness of the light guide plate is gradually reduced toward a rear side, and the first anisotropic diffusion surface is formed on the lower surface.
 7. The planar light source according to claim 6 further comprising a second anisotropic diffusion surface formed on a part of the upper surface of the light guide plate.
 8. The planar light source according to claim 1 wherein the first anisotropic diffusion surface comprises two kinds of anisotropic diffusion surfaces which are different from each other in characteristic of diffusion.
 9. The planar light source according to claim 1 further comprising a diffusion plate and light direction correcting means provided on the light guide plate.
 10. The planar light source according to claim 9 wherein the light direction correcting means comprises a Px prism sheet having X-direction prisms and a Py prism sheet having Y-direction prisms. 